Change Detection

In general, backup software doesn’t start copying files immediately when it runs. It must first figure out what to copy; that is, apart from the first full backup, it needs to know which files are new or different since it performed the last incremental update. A backup app typically starts each run by scanning all the folders and files you’ve asked it to back up, to determine not only what has changed but also how much space your backup will require and whether your backup will fit on the destination volume.

Some apps take a long time to scan, whereas others use any of several tricks to reduce or eliminate scanning time. Quite a few apps use the FSEvents (file system events) notification system in macOS to determine which files have changed recently, while others run in the background and use their own methods to watch for file changes. Whatever the mechanism, the result is that new or modified files can be copied immediately, and full scans (to identify changes that may have slipped through the cracks somehow) can occur infrequently. Among the backup apps that detect file changes instantly so that they can perform incremental updates without lengthy scans every time are Acronis Cyber Protect Home Office, QRecall, and Time Machine. Some other apps, like SuperDuper!, scan and copy in a single pass for greater efficiency.

Sources and Destinations

The volume from which you back up files is known as the source; the volume to which you back them up is known as the destination (or target). Be sure to select software that can accommodate the sources and destinations you want to use.

All backup apps can copy data from your startup disk, and most can also copy data from other attached hard drives and mounted network volumes. In most cases, your destination options also include any Finder-mountable volume. If you like, you can even back up your files onto a disk image (a special file that functions as a removable disk), although many apps require you to manually create the disk image using Disk Utility and mount it in the Finder before you can use it as a backup destination.

Some backup apps can copy data directly to online storage facilities, such as Amazon S3, Backblaze B2, a private server, or a proprietary cloud destination (Backblaze, IDrive, and the like). If you want to store a copy of your data online, you may find it convenient to use a single app for both local versioned backups and online backups, assuming you can find one that works with all your preferred destinations.

Rolling Backups

Among those backup apps that store multiple copies of your files, there’s an important distinction to make: true versioned backups versus rolling backups. In a true versioned backup, every version of every file you designate is saved, but identical files in the same location are never duplicated. In a rolling backup, the app creates a complete, separate copy of all your files each time it runs—basically a non-incremental backup. Then, after a certain number of days or backup runs (specified by the user), the app erases the oldest backup and adds a new one. Rolling backups give you multiple versions of all your files, but because they copy every single file each time they run, they take longer to perform and require much more storage space.

Versioned Backup Pruning

Whereas a rolling backup scheme saves a fixed number of complete backups, deleting older ones as newer ones are added, many backup apps offer a more sophisticated way of saving space: they prune (or erase) older files or snapshots from versioned backups when certain conditions are met. For example, an app might let you choose the maximum number of copies of any given file to save; once you reach that limit, it prunes the oldest one to make space for newer ones. Or it may go by age—extra versions older than, say, 30 days are deleted automatically to make space for newly backed-up files.

Time Machine does its own sort of pruning: it keeps hourly backups for 24 hours, daily backups for a week, and weekly backups until it runs out of space. That means almost every time it runs, it purges at least some older files. And when your backup disk is almost full, it deletes further files to ensure that, if possible, you always have at least a day’s worth of hourly backups and a week’s worth of daily backups—even if you’re seriously low on disk space.

Without a pruning feature, you could get stuck when your backup media runs out of space: you’d have to migrate your backups to a larger drive or manually erase backed-up files to make room for new ones. So pruning can be a valuable feature, but use it with caution; you don’t want to erase files you might need to recover later. All things being equal, I prefer to have a choice as to whether and when pruning should occur. So, especially when you’re considering an online storage provider for your backups, pay attention to the flip side of pruning: data retention. That is, be sure you know how long the service retains old versions and deleted files, and whether you can adjust the retention period.

File Format and Compression

To oversimplify somewhat, most software employs one of two basic methods to copy files when performing a backup. One way is to copy each file in a standalone Finder-readable format, so that the backed-up files look and act exactly like the originals. Another way is to copy all the files into a single, larger file (sometimes called an archive file or a backup set). Each approach has advantages and disadvantages.

Finder-format copies can be restored without backup software; just drag and drop or copy and paste. Some people feel more secure knowing they can get at their files easily even if their backup software isn’t working or the developer goes out of business. Generally, each version of each backed-up file takes up exactly as much space as the original.

Archive files, on the other hand, can be compressed as they’re stored, potentially saving a large amount of disk space. Of course, you’ll need the backup software to restore files, and you could have a slightly higher risk of data loss due to file corruption (since all the data is stored in a single file)—but most backup software has verification mechanisms to compensate for this.

Some backup software stores backups in disk images. Like archive files, disk images can thus contain many files and folders—and can optionally be compressed. But their contents are also Finder-readable.

For making a bootable duplicate, Finder-readable copies are obviously mandatory. For versioned backups, I consider Finder-readable files optional; the benefits of compression (along with delta encoding and deduplication, discussed ahead) that you can often get with proprietary archive files may outweigh the slight inconvenience of having to use backup software to restore your data. (Indeed, as I point out in Ease of Restoration, a backup app’s restore feature may turn out to be easier than using the Finder.)

Encryption

If there’s any chance at all that your backup drive could be lost, stolen, or otherwise accessible to an unauthorized person, you’ll be smart to encrypt its contents.

Some backup apps have built-in encryption features. You see this most often when an app stores backups in proprietary archive files; the archive itself is encrypted.

Encrypting Finder-format copies usually means encrypting the whole volume (using Disk Utility or a third-party tool). The same goes for disk images: they can be encrypted, which protects everything inside them. The contents of the encrypted volume (or disk image) will be protected only when it’s unmounted.

If you need an encrypted bootable duplicate, you can use either full-disk encryption software such as FileVault (which is built into macOS) or a hardware-encrypted drive, both of which I cover in the sidebar just ahead.

However, note that if you have a Mac with a T2 chip (see full list here), it can’t boot from an encrypted duplicate if that duplicate is formatted as Mac OS Extended (refer back to T2 Chips Change the Backup Rules), while if you have an M-series Mac you can’t currently boot from an encrypted duplicate at all (see M-Series Mac Changes). This is another argument against making a bootable duplicate at all.

Delta Encoding

In years past, almost all Mac backup software performed versioned backups on a file-by-file basis during incremental updates. In other words, if just 10 bytes of a 10 GB file change, that marks the file as modified, and thus the whole file must be copied on the next backup run. Increasingly, however, Mac backup tools (such as Backblaze, QRecall, Retrospect, and even macOS’s Versions feature; see Version Control) have a capability called delta encoding, which you may also hear referred to by numerous other terms. By whatever name, it means that the software copies only the changed portions of files.

In some cases, the software copies only the individual bytes that have changed since the last backup, and in other cases it copies larger units called blocks (a block being a unit of storage typically equal to 4096 bytes—4K—on modern Macs). So you’ll sometimes see this feature referred to as byte-level or block-level incremental updates, or words to that effect. With backup apps that use byte-level or block-level delta encoding, if only 10 bytes of a file change, only those 10 bytes, or the block(s) containing those 10 bytes, are added to the backup—a tiny amount of data.

The advantage of such an approach is that backups go much faster after the initial run and take up far less storage space. This is particularly important when backing up over the internet. The disadvantage is that restoring a file requires the backup software to reconstruct it by putting together the pieces from all its incremental backups. If even a single one of those incremental bits were to become damaged or lost, you might be unable to restore the file. However, backup software typically performs ongoing verification to ensure that all the necessary bits are present, alerting you or recopying data if any pieces go missing or become corrupted.

Deduplication

You might have two or more identical copies of a certain file on your disk. Some backup software notices this and puts only one copy in your backup (along with a record that the file appears in multiple places). That way, you save storage space and speed up your backups considerably. Taking this concept further, many backup apps can look within files for portions of files that are identical to portions of other files and, thanks to delta encoding, copy only the unique parts of the additional files. This process of preventing duplicate data (at any level) from cluttering up your backups is called deduplication. Deduplication applies only to versioned backups, not to duplicates (you can see the contradiction in the name!) and is extremely useful.

Almost every online backup app offers deduplication, which is great when you’re paying by the gigabyte or trying to push data over a slow internet connection. The result sometimes seems impossible: how did hundreds of megabytes of data just upload in a few seconds? That’s deduplication magic at work. QRecall and Retrospect are among the desktop backup apps that can also prevent duplicate data from appearing in your backups—even, in some cases, from multiple computers.

Selectors and Exclusions

Versioned backups may not include every file on your disk. If storage space is at a premium or if you want to save time on network backups, you might choose to include only part of your data in versioned backups (while putting all of it in your bootable duplicates). You can almost always do this manually, by selecting one or more specific files or folders to include or exclude. But some backup apps go further, letting you create patterns indicating which files or folders should be included (selectors) or excluded (exclusions) from a particular folder or volume based on items’ names, sizes, Finder labels, extensions, modification dates, and other factors. For example, you might want to include all Microsoft Word (.doc or .docx) files, regardless of their location, or exclude all files over 2 GB, even if they’re in a folder that’s otherwise backed up.

Snapshots

The term snapshot is important when it comes to backups, but it means different things in different contexts. In general, you should look for backup software that supports at least some variety of snapshots, but beyond that, the story gets a bit involved.

Snapshots in Backups

In the most general terms, you can think of a snapshot as a picture of what your entire disk (or at least the entire portion of the disk that you’re backing up) looked like at a moment in time. For example, suppose I back up my Mac every day, and every day some files are added, some are changed, and some are deleted. All the new and revised data gets added to my backup, but what if I want to see exactly what my disk looked like last Tuesday at 4:00? Well, if a backup occurred around that time, and my backup software supports snapshots, I could browse a hierarchical list of my files as they existed at that moment (each showing its size, creation and modification dates, and other metadata) and then decide whether to restore individual files or folders from that time, or even restore my entire disk to its state at that time.

By contrast, if it so happened that my Mac backed up only five files during that particular run (because only five had changed since the previous run), a backup app without snapshots might show me only those five files when I browse my backup from that time. So, if I wanted to return an entire folder or disk to its state at that time, I might have to laboriously work my way through many backups, selecting and restoring all the individual files that were backed up during each one.

Generically speaking, then, snapshots are useful because they show you how everything looked at various points in the past, not just the limited number of items that were backed up during a particular run.

When Time Machine first appeared, it introduced a novel way of creating snapshots without building a separate catalog of all your files: it relied on a Unix feature called a hard link (see the sidebar Understanding Hard Links), which gives the appearance of a file or folder existing in more than one place even though only one copy is taking up any real space. Several other Mac backup apps soon followed suit, using hard links to create snapshots. When backup software creates hard links to all the files or folders it didn’t copy in their entirety on a given run, you get a versioned backup that essentially functions as its own snapshot.

APFS Snapshots

Then APFS rolled around, and it has yet another concept of snapshots, which no longer relies on hard links. An APFS snapshot, which can be created in the blink of an eye by the file system itself, contains the metadata for all your files and folders, along with references to the physical space occupied by each of them on disk. It doesn’t contain copies of the files themselves, but it does contain pointers that tell macOS not to overwrite any of the blocks on disk occupied by the files it refers to. Individual snapshots are therefore comparatively small, though if you have a lot of them, their combined sizes can add up.

The effect is similar to that of other snapshot types: you can browse your entire disk as it appeared when a snapshot was taken and restore any individual file or folder, or you can restore the entire disk to its state at the time of a snapshot.

Ordinarily, your Mac creates an APFS snapshot once an hour, storing it on your startup volume; it keeps a limited number of these and then automatically purges older ones to save space. Although snapshots are a feature of APFS itself, they’re also an aspect of Time Machine. Time Machine creates the snapshots behind the scenes and also provides the user interface to browse and restore data from APFS snapshots, which it treats exactly like regular Time Machine backups. Indeed, when Time Machine does its normal backups, it begins by making an APFS snapshot on the destination drive.

You may be thinking, “Wait a minute! If my Mac creates APFS snapshots automatically, and if they function pretty much like Time Machine backups, why do I need Time Machine (or other backups) at all?” The first and most important reason is that APFS snapshots live on the same volume as the data itself. So if anything happens to the entire disk (or the entire computer), you’re out of luck. (As I’ve said a few times, an extra copy of your data on the same volume as the original isn’t really a backup at all.) APFS snapshots are, however, a convenience feature, letting you undo certain accidental deletions and changes without having to open a backup app or connect a backup drive. In addition, with regular backups, you have much more control over the number and frequency of snapshots, how often older backups are purged, and so on.

Nevertheless, numerous backup apps, including Carbon Copy Cloner, SuperDuper!, ChronoSync, and Acronis Cyber Protect Home Office, can create APFS snapshots identical to the ones Time Machine creates, and can restore data from any APFS snapshot (regardless of which app initiated it). In fact, in Monterey and later, even Disk Utility lets you view and mount snapshots, regardless of which app created them. Although I wouldn’t consider APFS snapshots obligatory, they are quite handy, and support for them in a backup app makes me happy.

Ease of Restoration

No matter how easy it is to back up your disk, if your software makes it difficult to restore files, you’re going to be unhappy with it. After all, a backup that you can’t restore is worthless. Backup apps typically offer one of three main approaches to restoration:

• A restore feature: The backup app tracks all the files you backed up during each session, allowing you to copy them back to their proper locations—or another destination of your choice—with a few clicks. In most cases, before starting the restoration, you can choose a subset of the files, or even pick out one version of a single file if that’s all you need. Restore commands and snapshots (of whichever sort) tend to appear together.

• Finder restoration: The backup app has no explicit restore feature; to restore files, you manually drag them (or copy and paste them) from the backup volume onto the original disk. This is fine for restoring an occasional file or folder if it’s in a convenient place, but if you’ve done a versioned backup, you may have to sort through dozens or hundreds of folders to locate the right version of each of your files.

• Reverse backup: In this scheme, the backup app once again lacks a Restore command; instead, it expects that you’ll swap the source and destination locations and perform your backup again—in reverse. While this may reduce manual effort somewhat, it’s still a hassle when restoring files from a versioned backup, especially when restoring from multiple locations.

I prefer apps with an explicit restore feature, as it usually makes restoration easier. Of course, the presence of a restore feature does not, by itself, mean the process will be easy (for example, some products have a Restore command that operates only at the level of individual files), but it’s a hopeful sign.

Bootable Duplicates with Versioning

Most backup apps that offer both versioning and bootable duplicates treat these two approaches as distinct modes, with the expectation that any given backup disk will hold one sort of backup or the other. But rather than storing two separate backups, wouldn’t it be nice if a single backup could be bootable and have the added bonus of containing multiple versions of all your files?

As a matter of fact, you can do this with several backup apps. But even though that may sound like the best of both worlds, it might not be what you’re looking for.

Here’s how such a scheme typically works: An app starts by making a bootable duplicate in the usual way (copying all files and attributes to give you an exact copy). On the next and subsequent runs, new and modified files are copied to their respective locations on the duplicate; however, deleted files and older versions, instead of being deleted from the duplicate, are moved to a special location on the backup disk—usually timestamped folders within a special folder—that won’t interfere with booting.

That’s nifty, no doubt about it. But in the apps I’ve seen that create bootable duplicates with versioning, the archive of older versions and deleted files is usually awkward to navigate and restore files from. It’s typically just a series of folders—perhaps hundreds or thousands of them—in the Finder. And there’s usually no restore feature to help you locate and retrieve those older files.

For that reason, I don’t recommend using a versioned bootable backup as a substitute for a conventional versioned backup. As a supplement to a conventional versioned backup, however—or as a substitute for a conventional bootable duplicate—I can’t object. If you’re creating versioned backups and bootable duplicates already, the additional safety net of versioned bootable duplicates can’t hurt.

Here are three apps I know of that offer bootable duplicates with optional versioning as I just described:

• Carbon Copy Cloner: My favorite app for bootable duplicates, Carbon Copy Cloner also has multiple ways of storing old versions of files, though they come with qualifications; see Carbon Copy Cloner Tips.

• ChronoSync: An all-purpose syncing and backup app, ChronoSync can archive changed and/or deleted files on the destination, whether or not you’re creating a bootable duplicate. In fact, starting with version 10, ChronoSync can store its archive of older and deleted files on a separate volume from your duplicate, if you prefer. (See also ChronoSync Tips.)

• Déjà Vu: The optional Safety Net setting in this backup app lets you store a user-selected number of archives of changed and deleted files in the destination folder of your choice—but as of publication time, Déjà Vu won’t run on Catalina or later.

Even if a backup app doesn’t offer a prebuilt “bootable duplicate with archived versions” option, you might be able to achieve the same effect with another app, by specifying the same destination volume for both bootable duplicates and versioned backups (with the versions stored in a folder at the top level of your backup disk). But because “bootable duplicate” normally means deleting anything on the destination that isn’t on the source, these two backup operations might conflict with each other.

A backup app called Mac Backup Guru, which is not yet compatible with Big Sur or later as of publication time, lets you combine a bootable duplicate (which it calls a Synchronized Clone Backup) with a versioned backup (or Backup with Incremental Snapshots). Like Time Machine with a Mac OS Extended destination, this app creates hard links (see Understanding Hard Links) to any files that are unchanged during a given backup run but creates complete copies of files that are new or different, and stores them all in timestamped subfolders of an Incremental Snapshots folder. The result is that the combination of bootable duplicate and versioned backup uses very little disk space, but restoring individual files is still awkward and Mac Backup Guru offers no automated Versioned Backup Pruning of older backed-up files.

Ease of Use

In addition to ease of restoration, an app’s overall ease of use is important. The interface should be self-explanatory—ideally, clear enough that you can figure out how to perform a basic backup and restoration without looking at a manual. (Time Machine stands out in this regard as being exceptionally easy to use, because it builds on the existing interface of the Finder rather than displaying your files in a completely different context. The downside, though, is that it’s impossible to restore files from multiple locations at the same time.) After initial setup, the best backup software is virtually invisible, working silently behind the scenes until you need it.

Price

The backup software included in the online appendixes ranges in price from free to about $120 (before discounts). The price doesn’t necessarily correlate to capabilities, but I urge you not to skimp when it comes to backup software; buy the solutions(s) that fit your particular scenario the best. Over the life of your data, the right backup software will surely pay for itself.

Arq

Arq is an unusual backup app designed to work primarily with cloud storage from providers such as Amazon (S3 or Glacier), Backblaze B2, Dropbox, Google Cloud Storage, Microsoft OneDrive, and Wasabi, although it can also back up to SFTP servers, NAS devices, and other local and network locations. Arq supports versioning, encryption, and file-level deduplication, and it faithfully backs up and restores all Mac metadata (such as file ownership and permissions, access control lists, extended attributes, Finder tags, and aliases)—a rare capability among online backup tools. On the downside, it backs up only on a fixed schedule (with a maximum frequency of once per hour). For more information, see Arq Tips.

Carbon Copy Cloner

My favorite tool by far for making bootable duplicates, lo these many years, Carbon Copy Cloner can also make versioned backups—but that’s a good option only if your startup volume and destination are both SSDs formatted as APFS. (See Carbon Copy Cloner Tips for details.) That arrangement lets you use a single backup volume for both bootable duplicates and Time Machine–compatible versioned backups without even partitioning the disk. (You’ll still want it to be reasonably large, however.)

ChronoSync

As the name suggests, ChronoSync is an app designed mainly for synchronization. For years, it’s been one of my favorite tools for syncing folders between my Macs. But it’s also a powerful backup tool that can create not only versioned backups but also bootable duplicates (including versioned bootable duplicates!). It can also store backups on Amazon S3 or Google Cloud servers. See ChronoSync Tips for additional usage hints.

QRecall

QRecall can save space by deduplicating files from more than one computer. It offers encryption, compression, and delta encoding, so that large files need not be entirely duplicated every time they change. QRecall works with external hard drives and network volumes, and it has a long list of clever, useful features. However, it can’t make bootable duplicates, and it has a peculiar interface that introduces several unusual terms (such as capture, recall, layer, and timeline) that force you to think about backups in unfamiliar ways. I help you make sense of these in QRecall Tips.

Retrospect

When I started writing books about backups more than 15 years ago, Retrospect was my favorite backup app, and I recommended it unreservedly. As time wore on and impressive competitors appeared—Time Machine, Backblaze, ChronoSync, and dozens of others—Retrospect’s status became more dubious as it stuck with an “old-school” approach to backups and an outdated interface. In recent years, however, Retrospect has enjoyed a comeback. It now has a much-improved interface; it also supports cloud storage destinations (such as Amazon S3, Backblaze B2, Google Cloud Storage, and Wasabi) and delta encoding (which Retrospect refers to as block-level incremental backups). I offer suggestions for using it in Retrospect Tips.

My main complaint was that Retrospect was more complex and expensive than most individual users would prefer, but then its target audience was business users with multiple computers (whether a few or thousands) to back up—and support personnel with enough technical expertise that they wouldn’t be bothered by a bit of fiddling.

That changed in 2018 with the release of Retrospect Solo, a new version of the app without some of the bells and whistles only a network administrator would want, and with the consumer-friendly price of $49 for a one-time purchase (or a subscription of $3.00 per month or $29 per year). Retrospect Solo can do just about any sort of backup an individual Mac user might want, except back up from a NAS (though it can use a NAS as a destination). You can find a complete feature comparison table on Retrospect’s website.

Retrospect Desktop, the next step up, can back up NAS devices just fine. It excels at client-server network backups (for both Macs and Windows PCs), for both small groups and large organizations. It can combine multiple hard disks into a single logical backup device—no need for a RAID. It also has good support for tape drives, magneto-optical drives, and other storage media that are common in large businesses. One copy of Retrospect Desktop includes five licenses for Retrospect Client—meaning it can back up the Mac it’s installed on plus up to five other computers. (Retrospect also makes editions of the app suitable for larger installations.)